The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to a method for producing high resolution NMR images with short scan times.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma. of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.z), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the z-y plane to produce a net transverse magnetic moment M.sub.xy, which is processing in the x-y plane at the Larmor frequency. The degree to which the net magnetic moment M.sub.z is tipped, and hence the magnitude of the net transverse magnetic moment M.sub.xy depends primarily on the length of time and the magnitude of the applied excitation field B.sub.1.
The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. In simple systems the excited spin induce an oscillating sine wave signal in a receiving coil. The frequency of this signal is the Larmor frequency, and its initial amplitude, A.sub.0, is determined by the magnitude of the transverse magnetic moment M.sub.xy. The amplitude, A, of the emission signal decays in an exponential fashion with time, t: EQU A=A.sub.0 e.sup.-t/T.spsp.*.sbsp.2
The decay constant 1/T.sup.*.sub.2 depends on the homogeneity of the magnetic field and on T.sub.2, which is referred to as the "spin-spin relaxation" constant, or the "transverse relaxation" constant. The T.sub.2 constant is inversely proportional to the exponential rate at which the aligned precession of the spins dephase after removal of the excitation signal B.sub.1 in a perfectly homogeneous field. As will be explained below, this characteristic is used in medical imaging to contrast tissues containing spins that exhibit different spin-spin relaxation times.
Another important factor which contributes to the amplitude A of the NMR signal is referred to as the spin-lattice relaxation process which is characterized by the time constant T.sub.1. This is also called the longitudinal relaxation process as it describes the recovery of the net magnetic moment M to its equilibrium value along the axis of magnetic polarization (z). The T.sub.1 time constant is longer than T.sub.2, much longer in most substances of medical interest. The T.sub.1 time constant is important in medical imaging because it determines the rate at which NMR measurements can be repeated without significantly degrading the NMR signal.
The NMR measurements of particular relevance to the present invention are called "pulsed NMR measurements". Such NMR measurements are divided into a period of excitation and a period of signal emission. Such measurements are performed in a cyclic manner in which the NMR measurement is repeated many times to accumulate different data during each cycle or to make the same measurement at different locations in the subject. A wide variety of preparative excitation techniques are known which involve the application of one or more excitation pulses (B.sub.1) of varying magnitude and duration. Such excitation pulses may have a narrow frequency spectrum (selective excitation pulse), or they may have a broad frequency spectrum (nonselective excitation pulse) which produces transverse magnetization M.sub.xy over a range of resonant frequencies. When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (G.sub.x, G.sub.y, and G.sub.z) which have the same direction as the polarizing field B.sub.0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
An image is reconstructed from the acquired NMR signals by performing a Fourier transform (FT). The preferred embodiment of the present invention employes a variant of the well known FT image reconstruction technique, which is frequently referred to as "spin-warp". The spin-warp technique is discussed in an article entitled "Spin Warp NMR Imaging and Applications to Human Whole-Body Imaging" by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980).
The spin-warp technique employs a pulse sequence in which a slice of spins are excited by applying a "selective" r.f. excitation pulse in the presence of a slice select magnetic field gradient (G.sub.z). In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by then applying a phase encoding gradient (G.sub.y) along that direction, and then a spin-echo NMR signal is acquired in the presence of a readout magnetic field gradient (G.sub.x) in a direction orthogonal to the phase encoding direction. The readout gradient present during the NMR signal acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse G.sub.y is incremented (.DELTA.G.sub.y) in a sequence of views that are acquired during the scan to produce a set of NMR data from which an entire image can be reconstructed.
In a conventional spin-warp NMR data acquisition the spin-echo NMR signal is centered in the data acquisition window as illustrated in FIG. 4A. Typically, this centered echo signal is sampled either 128 times or 256 times within the acquisition window. The digitized samples are stored as complex numbers in a two-dimensional array in which each row in the array is one of the digitized spin-echo NMR signals. An image is reconstructed by performing a 2D Fourier transformation of the data in this array after all of the values therein have been acquired during the scan.
There are a number of factors in this NMR method which influence the quality of the reconstructed image: signal-to-noise ratio (SNR); resolution; and field of view (FOV). These factors are greatly influenced by the manner in which the NMR signal is acquired and trade-offs are required in any particular acquisition. For example, the SNR of the acquired NMR signal can be improved if the rate at which the NMR signal is sampled and digitized (i.e. receiver bandwidth) is reduced, but this results in either a decrease in image resolution because fewer samples are taken, or an increase in total scan time because the sampling must extend over a longer time period. Similarly, the minimum FOV of the image is a function of the receiver bandwidth and the readout gradient strength, and as a result, when the receiver bandwidth is reduced to improve SNR, it results in an enlargement of the FOV of the image unless the readout gradient strength is reduced by a proportional amount.
When an image is reconstructed using a conventional 2D Fourier transformation, it is imperative that the array of acquired NMR data be complete. Lack of data, or corrupted data will produce artifacts in the reconstructed image. As disclosed by D. C. Noll, D. G. Nishimura and A. Macovski in "Homodyne Detection in Magnetic Resonance Imaging", IEEE Trans. on Med. Image., May 9, 1990, however, methods are available for calculating, or filling in, some missing data. This homodyne reconstruction technique is used, for example, to reduce the number of views acquired during a scan in order to reduce total scan time. The homodyne reconstruction technique "fills in" the missing views so that a quality image can be reconstructed. The technique has also been used to shorten the pulse sequence echo time (TE) by moving the echo peak forward by an amount .DELTA.TE in the data acquisition window as illustrated in FIG. 4B.